CNN's Nick Glass explores whether or not Leonardo da Vinci's famous painting should be cleaned.

About 106.5 million people visited the top 20 museums in the world last year, a slight drop from 107.3 million in 2014, according to the index. Fluctuation in museum attendance numbers is often driven by popular temporary exhibits, such as Tate Modern's "Henri Matisse: The Cut-Outs" in 2014.

London had six museums in the top 20, the most of any city. Washington, D.C., came in second place with four, while Paris and New York tied for third place with two each.

World's 20 most-popular museums

1. Louvre, Paris, France (8,700,000 visitors)

2. National Museum of China, Beijing, China (7,290,000)

3. Smithsonian National Museum of Natural History, Washington, D.C., United States (6,900,000)

3. Smithsonian National Air and Space Museum, Washington, D.C., United States (6,900,000)

5. British Museum, London, United Kingdom (6,821,000)

6. The Metropolitan Museum of Art, New York, United States (6,300,000)

7. Vatican Museums, Vatican, Vatican City (6,002,000)

8. Shanghai Science & Technology Museum, Shanghai, China (5,948,000)

9. National Gallery, London, United Kingdom (5,908,000)

10. National Palace Museum (Taiwan), Taipei, Taiwan (5,288,000)

11. Natural History Museum, London, United Kingdom (5,284,000)

12. American Museum of Natural History, New York, United States (5,000,000)

13. Tate Modern, London, United Kingdom (4,713,000)

14. National Gallery of Art, Washington, D.C., United States (4,104,000)

15. National Museum of American History, Washington, D.C., U.S. (4,100,000)